Depth filter media

ABSTRACT

A porous filter medium is formed by a pleated cylinder of a depth filter medium. An absolute rated inner filtration layer is arranged between upstream and downstream drainage layers. The depth filter medium is formed from a continuous sleeve. Additionally or alternatively at least the downstream drainage layer may be formed integrally with at least a part of an inner filtration layer. The material of the medium may be a fibrous mass of non-woven synthetic polymeric micro-fibers which are free of fiber-to-fiber bonding and are secured to each other by mechanical entanglement or intertwining, with the diameter of the fibrous structure varying across the medium.

BACKGROUND TO THE INVENTION

1. Field of the Invention

The invention relates to depth filter media.

In this specification "depth filter" means a filter with pores capableof removing from a fluid particles that may be smaller than the size ofpores in the filter, the particles being trapped by progressiveinterception during changes of direction of the pores. Such depthfilters have a high dirt capacity.

Depth filters are commonly utilized in the form of a thick continuouscylinder of filter medium that surrounds a central core and may beprovided with an external cage. Such depth filters have, because oftheir low external area and great depth (typically about 15 mm) arelatively high pressure drop when a fluid is passed through the filter.On the other hand, as mentioned above, such filters have relatively highdirt capacity, because of the high internal void space in whichcontaminant can accumulate.

2. Brief Review of the Prior Art

GB-A-585295 discloses a filter element formed by an elongated web ofcellulose filter material folded to extend back and forth between theinner and outer surfaces of the tubular body. The folds extendlongitudinally and a tubular binding strip is secured to the innerfolds. No drainage layer is provided. GB-A-1389199 discloses a filterformed by corrugating a flat sheet felt medium backed by woven wire. Theends of the sheet are sealed after pleating. The woven wire supports thefelt and no drainage layer is provided.

GB-A-1460925 discloses a filter element formed from a corrugated sheetof depth filter medium in which the edges of the sheet are connectedtogether after corrugation. Separate upstream and downstream protectivelayers are provided. U.S. Pat. No. 4,233,042 discloses a pleated filtermedium formed from sheets of glass fibre. EP-A-0083789 discloses apleated sheet of filter material such as a fibrous filter material. Theedges cf the sheet are side-sealed after corrugation.

Although pleating increases the surface area per unit volume of filtermaterial, and thus increases the dirt capacity, the presence of sideseals formed after pleating can be disadvantageous. First, the presenceof the seal forms an obstruction to uniform flow through the filterelement. Secondly, the formation of the side seal requires additionalmanufacturing steps after pleating (see GB-A-1460925). Thirdly, thepresence of a side seal forms a potential leakage path because completesealing is difficult.

In order to achieve optimum performance of a pleated filter, it isnecessary to provide a relatively coarse upstream drainage layer toallow drainage of fluid down between the pleats and to give a void spacefor the accumulation of dirt. They also support the filter medium. It isalso necessary to provide a relatively coarse downstream drainage layerto allow filtrate to drain between the pleats from an inner filtrationlayer and also to support the filter medium against applied pressure.

The drainage layers are conventionally formed separately from the filtermedium by layers of non-woven fabrics or nets located on either side ofthe filter medium (see GB-A-1460925). The provision of such separatelayers and their attachment to the filter medium complicates themanufacture of the filter and increases its cost.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a filterelement comprising a generally cylindrical depth filter medium formed byat least one continuous sleeve of filter medium provide with pleatsextending along the length of the filter medium and parallel to oneanother around the filter medium, an inner support core within thepleated depth filter medium and contacting inward ends of the pleats andan outer support cage contacting outer ends of the pleats.

According to a second aspect of the invention, there is provided afilter element comprising a generally cylindrical depth filter mediumincluding an inner filtration layer arranged between upstream anddownstream drainage layers, the downstream drainage layer being formedintegrally with at least a part of the inner filtration layer and havingan absolute rating that is greater than the absolute rating of the innerfiltration layer, the depth filter medium being provided with pleatsextending along the length of the filter medium and parallel to oneanother around the filter medium, an inner support core with thecylindrical depth filter medium contacting inner pleats and an outersupport cage contacting outer pleats.

According to a third aspect of the invention, there is provided a methodof manufacturing a filter element comprising forming at least onecontinuous sleeve of depth filter medium, pleating the sleeve to form aplurality of pleats extending along the length of the filter medium andparallel to one another around the filter medium, arranging an innersupport core within the pleated depth filter medium contacting inwardends of the pleats and arranging an outer support cage in contact withouter ends of the pleats.

According to a fourth aspect of the invention, there is provided amethod of manufacturing a filter element comprising forming a generallycylindrical depth filter medium including an inner filtration layer,arranged between upstream and downstream drainage layers, forming thedownstream drainage layer integrally with at least a part of the innerfiltration layer and with an absolute rating that is greater than theabsolute rating of the filtration layer, pleating the depth filtermedium to provide pleats extending along the length of the filter mediumand parallel to one another around the depth filter medium and thenproviding an inner support core within the pleated depth filter mediumand contacting inner pleats and an outer support cage contacting outerpleats.

By forming the drainage layers integrally with at least a part of theinner filtration layer the dirt capacity and pressure drop can beimproved. The ease and cost of manufacture are also improved.

The following is a more detailed description of an embodiment of theinvention, by way of example, reference being made to the accompanyingdrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a cylinder of depth filter medium prior topleating and formed as a continuous cylinder,

FIG. 2 is a perspective view of a cylinder of depth filter medium priorto pleating and formed from a rectangular sheet of medium with oppositeedges sealed together by a side seal,

FIG. 3 is a cross-sectional view of a cylinder of depth filter mediumprior to pleating and formed from a rectangular sheet of medium rolledso that opposed edges of the sheet overlap at least once,

FIG. 4 is a cross-section through a filter element including a depthfilter medium.

FIG. 5 is a schematic cross-section of part of a first pleated cylinderof depth filter medium of the kind shown in FIG. 4 and provided withintegral upstream and downstream drainage layers,

FIG. 6 is a graph plotting Bubble Point pressure against distance asmeasured from the upstream drainage layer in a direction normal to thesurface of the upstream drainage layer, for first and second forms offilter of the kind shown in FIG. 5,

FIG. 7 is a graph plotting Bubble Point pressure against distance asmeasured from the upstream drainage layer in a direction normal to thesurface of the upstream drainage layer, for a third form of filter ofthe kind shown in FIG. 5,

FIG. 8 is a schematic cross-section of part of a second pleated cylinderof depth filter medium of the kind shown in FIG. 4 and provided withintegral upstream and downstream drainage layers, and

FIG. 9 is a graph plotting Bubble Point pressure against distance asmeasured from the upstream drainage layer in a direction normal to thesurface of the upstream drainage layer, for first and second forms offilter of the kind shown in FIG. 8,

FIG. 10 is a graph plotting Bubble Point pressure against distance asmeasured from the upstream drainage layer in a direction normal to thesurface of the upstream drainage layer for a third form of filter of thekind shown in FIG. 8,

FIG. 11 is a schematic cross-section of part of a depth filter, prior topleating and formed by an outer sleeve of polypropylene fibres and aninner sleeve of glass fibre membrane formed into a roll,

FIG. 12 is a schematic cross-section of part of a depth filter prior topleating and formed by an outer sleeve of polypropylene fibres, acentral sleeve of a glass fibre membrane formed into a roll and an outersleeve formed from a sheet of nylon or polyester non-woven fibres formedinto a roll, and

FIG. 13 is a schematic cross-section of part of a depth filter prior topleating and showing an outer sleeve in accordance with FIG. 5 or FIG. 6combined with an inner sleeve formed by a nylon membrane.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIGS. 1 to 4, the filter element comprises agenerally cylindrical depth filter medium 10 provided with pleats 11.Each pleat extends along the length of the filter medium and the pleatstogether extend parallel to one another around the filter. An innersupport core 12 is arranged within the cylindrical depth filter mediumand contacts inner pleats. An outer support cage 13 contacts the outerpleats.

The cylinder of depth filter medium 10 may be a continuous sleeve offilter medium without any side seal. This is shown in FIG. 1. A secondpossibility is to form the cylinder of filter medium 10 from a flatsheet of depth filter medium that has been sealed with a seal 10a toform a cylinder. This is shown in FIG. 2. A third possibility is to rolla flat rectangular sheet into a cylindrical roll with opposed edges ofthe sheet overlapping at least once. This is shown in FIG. 3.

The filter medium may be a fibrous structure such as polyolefins,polyesters, polyamides, glass fibres, cellulose fibres or metal fibres.For example, the depth filter element may be formed from fibres producedby a melt blowing process and having diameters from 1-20 micrometers,preferably 1-12 micrometers. Such a melt blowing process is the subjectof U.K. Patent Application Publication No. 2152471A.

The voidage of the depth filter medium may be constant throughout itsdepth.

The overall length of the depth filter medium 10 may be from 1-250 cmsand preferably from 2-100 cms. The outside diameter of the depth filtermedium 10 may be between 2-100 cms and is preferably between 4-40 cms.In the embodiment illustrated in FIG. 4, there are 16 pleats, but theremay be between 6-300 pleats, preferably between 8-32 pleats. The pitchof the pleats may be between 2-50 mm but is preferably between 4-15 mm.The depth of the pleats may be from 4-50 mm and preferably 5-25 mm.

The depth filter medium may have a thickness of between 0.5-25 mm andpreferably between 1-25 mm. The ratio of the surface area per unitlength to the depth may be from 20 to 2000 and is preferably from 40 to200.

The fibres may be randomly directed or may be directed with an elementof circumferential or axial orientation.

Referring next to FIG. 5, a first form of pleated cylinder of the kinddescribed above with reference to

FIG. 4 comprises a filter medium made by the method disclosed inGB-A-2152471 so that it is formed from a fibrous mass of non-woven,synthetic, polymeric micro-fibres, the micro-fibres being substantiallyfree of fibre to fibre bonding and secured to each other by mechanicalentanglement or intertwining. The fibrous mass has a substantiallyconstant voids volume over at least a substantial portion thereof asmeasured in the radial direction.

As not disclosed in that specification, however, the fibrous mass of thefilter of FIG. 4 is formed with outer and inner portions 14,15 of fibreswhich have a greater diameter than the fibres of the inner filtrationportion 16. Thus, the outer and inner portions are comparatively coarseand provide upstream and downstream drainage layers 14,15 respectivelywhile the central portion provides the absolutely rated inner filtrationlayer 16 of the filter medium.

The diameters of the fibres may be varied in a number of different ways.Firstly, the diameter of the fibres may be decreased gradually from theouter surface of the upstream drainage layer 14 to the centre of theinner filtration portion 16 and then may be increased from the centre ofthe inner filtration portion 16 to the surface of the downstreamdrainage layer 15. Secondly, the diameter of the fibres may be generallyuniform in the upstream and downstream drainage layers 14,15 and thediameter of the fibres may be generally uniform in the inner filtrationportion 16, this diameter being less than the diameter of the fibres inthe upstream and downstream drainage layers 14,15. Of course, thediameter of the fibres may also be varied in a combination of the waysdescribed above. As a third example, the diameter of the fibres maydecrease from the outer surface of the upstream drainage layer 14 andmay then have a generally constant smaller diameter in the innerfiltration portion 16 before increasing to the surface of the downstreamdrainage layer 15.

One measure of pore size is the Bubble Point test. In such a test,successive sections of the filter medium across its thickness aresubmerged in a liquid bath to wet out all the pores. Pressure is thenapplied to the interior of the structure and the pressure required forthe first or initial bubble of air to appear on the exterior surface ofthe cylinder is recorded. FIG. 6 is a graph plotting the Bubble Pointpressure (i.e. the pressure required for the first or initial bubble ofair) against the distance as measured from the outer drainage layer 10in a direction normal to the surface of the layer 10 across the sectionX--X of the filter medium of FIG. 5 for the first and secondconstructions of this embodiment referred to above. The continuous lineshows the Bubble Point pressure pressure variation for the uniformlydecreasing and increasing fibre diameter and the broken lines show theBubble Point pressure variation for the portions of constant fibrediameter. In both cases the voidage is constant as described above. Asshown in continuous line, the Bubble Point pressure may increase fromthe upstream drainage layer 14 to a maximum towards the core 16 of thefilter medium within the absolutely rated layer and then decrease to thedownstream drainage layer 15. The lower Bubble Point pressures in theupstream and downstream drainage layers 14,15 indicate that the poresize in these regions is greater than the pore size in the absoluterated central layer 16.

In the alternative embodiment illustrated in broken line, the pore sizeis discontinuous between the layers, being greater in the upstream anddownstream drainage layers 14,15 than in the absolute rated centrallayer 16.

FIG. 7 shows the Bubble Point pressure distribution for the thirdconstruction described above. The Bubble Point pressure increases fromthe outer surface of the upstream drainage layer 14 to a generallyconstant value in the inner filtration portion 16 before decreasing tothe surface of the downstream drainage layer 15.

Referring next to FIG. 8, the second form of pleated filter medium isformed by two contiguous sleeves of filter medium. The outer sleeveforms the upstream drainage layer 14 and a first portion of theabsolutely rated inner filtration layer 16 while the inner sleeve formsthe other portion of the absolutely rated inner filtration layer 16 andthe downstream drainage layer 15. Each sleeve may be formed as describedin examples 11,12,13,47 and 48 of GB-A-2152471. One sleeve is thenturned inside out so that the coarser outer layer becomes the downstreamdrainage layer 15 and so forms the inner sleeve. This inner sleeve isthen inserted into the outer sleeve to give the structure shown in FIG.8. Of course, the whole of the inner filtration layer 16 may be providedby only one of the sleeves (either the inner or the outer). Of course,the requisite structure for the inner sleeve could be formed by themethod described and so obviate the need to turn inside a sleeve ofreverse structure.

The diameters of the fibres making up the sleeves may be varied asdescribed above with reference to FIGS. 5 and 6. The upstream anddownstream drainage layers 14,15 may be of constant fibre diameterthroughout their depth to give a uniform pore size with the compositeabsolutely rated core layer also being of constant, finer, pore sizeacross its depth being formed from fibres of a uniform diameter which issmaller than the diameter of the fibres of the upstream and downstreamdrainage layers.

Secondly, the pore size may vary continuously from a coarser to a finervalue from the upstream drainage layer 14 to the centre of theabsolutely rated inner filtration layer 16 and then from a finer to acoarser value from the centre of the absolutely rated core layer to thedownstream drainage layer 15. As described above, this is achieved bydecreasing the diameter of the fibres from the upstream drainage layer14 to the centre of the inner filtration layer 16 and then increasingthe diameter of the fibres to the surface of the downstream drainagelayer 15.

FIG. 9 is a graph plotting the Bubble Point pressure against thedistance across the filter medium of FIG. 8 as measured from theupstream drainage layer 14 in a direction normal to the surface of theupstream drainage layer 14 and for these two constructions. Thecontinuous line shows a filter medium in which the pore size variescontinuously from a maximum at the surface of the upstream drainagelayer 14 to a minimum at the centre of the absolutely rated innerfiltration layer 16 and then to a maximum at the surface of thedownstream drainage layer 11. The broken line shows coarser pore sizesin the upstream and downstream drainage layers 14,15 and a constantfiner pore size in the absolutely rated inner filtration layer 16.

Thirdly, the fibre diameter may be varied so that the pore size varieswith the diameter of the fibres decreasing from the surface of theupstream drainage layer 14 to a constant smaller diameter in the firstportion of the inner filtration layer 16. The second portion of theinner filtration layer 16 then has a constant smaller diameter (which isgreater than the first constant smaller diameter) and then increases indiameter to the surface of the downstream drainage layer 15.

FIG. 10 is a graph plotting the Bubble Point pressure against thedistance across this third form of the filter medium of FIG. 8 asmeasured from the upstream drainage layer 14 in a direction normal tothe surface of the upstream drainage layer.

It will be appreciated that the downstream drainage layer need not beformed integrally with the absolutely rated inner filtration layer, itcould be formed by a separate layer of depth filter medium, or membraneor drainage net. It will also be appreciated that, where the innerfiltration layer 16 has an absolute rating of, for example, 20 mm, adownstream drainage layer 15 having an absolute rating of 90 mm willprovide an `open` structure in comparison with the inner filtrationlayer 16.

EXAMPLE I

Seven filter media were prepared as described above with reference toFIG. 8 and having the structure described below in Table 1. In eachcase, the structure was formed from two sleeves. One of the sleeves hada decreasing pore size from the upstream surface to the downstreamsurface, the pore size being open (0) at the upstream surface and havingan absolute rating (T) in a zone adjacent the downstream surface.(Referred to as O-T, indicating that the pore structure is open upstreamand tight downstream). The other sleeve had an increasing pore size fromthe upstream surface to the downstream surface. Thus, this sleeve hadthe absolute rated pore size (T) in a zone adjacent the upstream surfaceand was open (O) at the downstream surface. (Referred to as T-Oindicating that the pore structure is tight upstream and opendownstream). The "absolute rating" of a filter medium is determined asdescribed below.

The inner filtration layer will be formed by the zone or zones of thesleeve or sleeves which have the lowest absolute rating.

Seven filter media were prepared having the following structure:

                  TABLE 1                                                         ______________________________________                                                        Absolute           Absolute                                                   Rating of          Rating of                                                  Upstream           Downstream                                        Upstream Structure Downstream                                                                             Structure                                  Filter No                                                                            Structure                                                                              (mm)      Structure                                                                              (mm)                                       ______________________________________                                        1      O - T     5        T - O    90                                         2      O - T    20        T - O    20                                         3      O - T    20        T - O    90                                         4      O - T    40        T - O    40                                         5      O - T    90        T - O    40                                         6      O - T    70        T - O    70                                         7      O - T    90        T - O    90                                         ______________________________________                                    

Each filter was nominally 254 mm long with 16 pleats and an outsidediameter of 70 mm. The thickness of the medium was from 3-4 mm and theoutside diameter of the inner core 35 mm.

The seven filter media were then tested for pressure drop by passing onehundred liters per minute of clean water through each filter medium andmeasuring the pressure drop in millibars. Next, the dirt capacity ofeach filter medium was measured by using a modified form of the OSU F2test developed at Oklahoma State University. In this test, astandardised silicaceous contaminant, AC fine test dust, is prepared asa stable suspension of known weight in water. This suspension is thenpumped at 10 liters per minute through filter medium. The test system isequipped with two particle counters each with a range of 3 to 100microns. One counter, upstream of the filter, records the influentparticle levels and the other downstream similarly record the effluentparticle levels. The samples are analysed by the counters for theircontent of particles greater than five or more different preselectedparticle diameters and the ratio of the upstream count to the downstreamcount is automatically recorded. Simultaneously the pressure drop acrossthe test filter is measured as the test suspension flowed through thefilter and is recorded as a function of time. The quantity ofcontaminant (in grammes) incident on the filter required to develop adifferential pressure of 40 psi (2.8 kg/cm²) is recorded as the dirtcapacity of the filter medium.

The pressure drop was then compared with the pressure drop of a knownnon-pleated depth filter ("non-pleated") having generally the samelength and the same outside diameter as the filter medium under test andgenerally the same absolute rating. The known non-pleated depth filterused for comparison was thicker than the filter media of Nos. 1 to 7.The results of these tests are shown in Table 2.

The "absolute rating" is determined from the ratio measured by thecounters, which is known as the beta ratio and provides the removalefficiency at each of the preselected particle diameters.

The beta ratio for each of the five or more diameters tested is plottedas the ordinate against particle diameter as the abscissa, usually on agraph in which the ordinate is a logarithmic scale and the abscissa is alinear scale. A smooth curve is then drawn between the points. The betaratio for any diameter within the range tested can then be read fromthis curve. Efficiency at a particular particle diameter is calculatedfrom the beta ratio by the formula:

    Efficiency, percent=100 (1-1/beta).

As an example, if beta=1000 efficiency=99.9 percent.

The absolute removal ratings cited in the examples presented below arethe particle diameters at which beta equals 5,000 and the efficiency is99.98 percent.

It will be seen that the seven filter media of the kind described abovewith reference to FIG. 8 have significantly improved pressure drops incomparison with similarly rated non-pleated filters of similar lengthand diameter (e.g. the "non-pleated" filter with a pressure drop of 1500mbar has a rating equivalent to the rating of Filter No.1). This is due,at least in part, to the higher surface area per unit volume, thereduced thickness, the absence of side seals and the presence of adrainage layer integral with a part of the inner filtration layer. Inaddition, the dirt capacities of the seven filter media described abovewith reference to FIG. 8 are high.

                  TABLE 2                                                         ______________________________________                                                                "Non-Pleated"                                                                 Pressure drop                                         Invention of FIG. 8     at 100 LPM                                                            Pressure drop     clean water                                                 at 100 LPM        flow rate at                                       Absolute clean water                                                                              Dirt   equivalent                                         Rating   flow rate  Capacity                                                                             rating                                      Filter No                                                                            mm       mbar       grammes                                                                              mbar                                        ______________________________________                                        1        4.3    285          17.5 1500                                        2        10.8   107        36     660                                         3        14.8   67         28     280                                         4      26       73         66     180                                         5      40       41         67     180                                         6      70       31         79     145                                         7      90       35         111     90                                         ______________________________________                                    

EXAMPLE 2

Three filter media were prepared having the structure given below inTable 3, using the same notation described above with reference to Table1.

                  TABLE 3                                                         ______________________________________                                                Pleated(Y)/         US           DS                                   Filter No                                                                             Unpleated(N)                                                                             US       AR    DS     AR                                   ______________________________________                                        8       N          --       5     --     --                                   9       Y          O - T    5     --     --                                   10      Y          O - T    5     T - O  90                                   ______________________________________                                         US = Upstream Structure                                                       DS = Downstream Structure                                                     AR = Absolute Rating (mm)                                                     Filter No. 10 is the equivalent of Filter No. 1 of Table 1.              

The media of Nos. 8 to 10 were then tested for pressure drop and dirtcapacity by the methods described above with Table 2. The results wereas follows:

                  TABLE 4                                                         ______________________________________                                                             Flow (l/min)                                                      Pressure    at which   Absolute                                               Drop        pressure drop                                                                            Rating                                        Filter No                                                                              (mbar)      measured   (mm)                                          ______________________________________                                        8        1500        100        5                                             9        840          60        5                                             10       285         100          4.3                                         ______________________________________                                    

It will be seen that, in comparison with the unpleated filter of No. 8,the pleated filter of No. 9 has a significantly lower pressure drop. Theaddition of a downstream drainage layer of depth filter medium (No. 10)produces a further significant reduction in pressure drop.

A further improvement in pressure drop can be achieved by providing thefilter media with upstream and downstream drainage nets. Thee drainagenets are open mesh nets of polypropylene in close contact with theadjacent surface of the media. Their effect is most pronounced withfilter media having lower absolute ratings. The nets preferably have amesh with diamond-shaped openings.

For example, providing the filter media No.2 of Table 2 and No.9 ofTable 3 with upstream and downstream drainage nets had the followingeffect on pressure drop.

                  TABLE 2A                                                        ______________________________________                                        Pressure drop (mbar)                                                                                       Clean water flow rate                                                         (lpm) at which pressure                          Filter No                                                                             Without Nets                                                                             With Nets drop measured                                    ______________________________________                                        2       107         95       100                                              9       840        100        60                                              ______________________________________                                    

The mesh provides the filter medium with drainage channels that improvethe flow of fluid through and from the filter.

Under aggravated conditions, the provision of nets can also improve dirtcapacity. For example, under conditions simulating those found in thefiltration of metal oxide dispersions used in magnetic tape manufacture,the dirt capacity of filter media provided with nets is greater than thedirt capacity of otherwise identical filter media but omitting nets.

For example, the filter medium No. 2 of Table 2 was modified byproviding upstream and downstream drainage nets. The modified media wasthen compared with unmodified media in the filtration of a suspension ofFine Test Dust supplied by the AC Spark Plug Company (ACFTD) in tetrahydrofuran (THF) with a polyurethane viscosifier to raise the viscosityto about 2 poise, and gave the following result:

                  TABLE 2B                                                        ______________________________________                                        Dirt Capacity in grammes ACFTD in THF(viscosified)                            Filter No     Without Nets                                                                             With Nets                                            ______________________________________                                        2             27         38                                                   ______________________________________                                    

It will thus be seen that the provision of drainage nets provides animprovement in dirt capacity. It is presently believed that theimprovement arises because the nets prevent or limit swelling and soprovide dimensional stability.

EXAMPLE 3

Two filter media were prepared having the structure given below in Table5, using the same notation as described above with reference to Table 1.

                  TABLE 5                                                         ______________________________________                                                          Absolute          Absolute                                                    Rating of         Rating of                                                   Upstream          Downstream                                       Upstream   Structure                                                                              Downstream                                                                             Structure                                 Filter No                                                                            Structure  (mm)     Structure                                                                              (mm)                                      ______________________________________                                        11     O - T, O - T                                                                             90, 20   T - O    90                                        12     O - T      20       T - O    90                                        ______________________________________                                         Filter No. 12 is the equivalent of Filter No. 3 of Table 1.              

The media of Nos. 11 and 12 were then tested for pressure drop and dirtcapacity by the method described above with reference to Table 2. Theresults were as follows:

                  TABLE 6                                                         ______________________________________                                                          Flow (l/min)                                                        Pressure  at which   Absolute                                                                              Dirt                                             Drop      pressure drop                                                                            Rating  Capacity                                 Filter No                                                                             (mbar)    measured   (mm)    (grammes)                                ______________________________________                                        11      62        100        11.6    37                                       12      67        100        14.6    28                                       ______________________________________                                    

In this Example, Filter No. 11 was formed from three layers. The centrallayer has the lowest absolute rating and thus forms the filtrationlayers. The upstream and downstream layers form upstream and downstreamdrainage layers respectively.

In comparison with the two layer structure of No. 12, the three layerstructure of No. 11 shows extra drainage, better (reduced) pressure dropand increased dirt capacity. In addition, although the absolute ratingof the filtration layer is nominally the same in both filters, theabsolute rating of the complete media is less in No. 11 than in No. 12.

EXAMPLE 4

Three pleated cylindrical filter media were prepared having a structuredescribed above with reference to FIGS. 5 and 6 in which each filtermedium is formed from a single mass of fibres where the diameter of thefibres decreases gradually from the outer surface of the upstreamdrainage layer 14 to the centre of the inner filtration portion 16 andthen increases from the centre of the inner filtration portion to thesurface of the downstream drainage layer. Using the notation introducedabove in Example 1, this gives an O-T-O structure (open-tight-open). Theabsolute rating of the filtration portion is given in Table 7 below.

The filter media were provided with upstream and downstream open meshpolypropylene nets.

The dirt capacity and the pressure drop were then tested as describedabove. The results are given in Table 7 below.

                  TABLE 7                                                         ______________________________________                                                                            Pressure                                                                      drop at                                                                       100 lpm                                                     Absolute  Dirt    clean water                                                 Rating    Capacity                                                                              flow rate                                 Filter No                                                                             Structure (mm)      (grammes)                                                                             (mbar)                                    ______________________________________                                        13      O - T - O 12        40      100                                       14      O - T - O 20        36      45                                        15      O - T - O 40        100     31                                        ______________________________________                                    

It will be seen that in comparison with the equivalent "non-pleated"filter media of Table 2, the media of Table 7 give reduced pressuredrops. In comparison with the equivalent (or generally equivalent)filter media according to the invention of FIG. 8 in Table 2, the mediaof Table 7 give improved dirt capacity.

It will, of course, be appreciated that the depth filter medium need notbe made by the method disclosed in GB-A-2152471. The depth filtermedium, as described above, can be made by any suitable method.

In the depth filter medium of the kind described above in Example 1(where the media is formed by two sleeves of media made by the methoddisclosed in GB-A-2152471) or in filter No.11 of Example 3, where threesuch sleeves are provided, the sleeves need not all be of the samematerial. They may be of any suitable depth filter medium such as afibrous filter material.

In an alternative arrangement, in the filters of Example 1, the outersleeve may be formed by a continuous cylinder 20 of polypropylenefibrous depth filter medium, as described, but the inner sleeve may beformed of a rectangular sheet 21 of glass fibre membrane formed into acylinder by being wrapped into a roll with opposite edges of the sheetoverlapping each other at least once. This is shown in FIG. 11. Thecylinders are then pleated.

EXAMPLE 5

Two pleated filter media were prepared. Both media were formed from twolayers of material arranged as described above with reference to FIG. 8.In both cases, the outer layer of the filter media comprised theupstream structure of Filter No.1 of Table 1--an O-T structure ofpolypropylene fibres having an absolute rating of 5 mm. In both casesthe downstream layer was formed by a resin bonded glass fibre mediumhaving the rating given in Table 8 below sold by Pall Corporation underthe trade mark ULTIPOR GF. In both cases, the glass fibre medium isbound by a binder resin which coats the glass fibres, imparting apositive charge (zeta potential) to the medium. In the medium of FilterNo.17 of Table 8, the positive zeta potential was greater than in FilterNo.16 of Table 8. Both filters were provided with upstream anddownstream nets.

                  TABLE 8                                                         ______________________________________                                        Upstream Structure  Downstream Structure                                      Filter             Absolute         Absolute                                  No.    Material    Rating   Material                                                                              Rating                                    ______________________________________                                        16     Polypropylene                                                                             5 mm     Glass fibre                                                                           1 mm(approx)                                                          (positively charged)                              17     Polypropylene                                                                             5 mm     Glass fibre                                                                           1 mm(approx)                                                          (more highly positively                                                       charged than No. 10)                              ______________________________________                                    

Filters Nos.16 and 17 were then tested for pressure drop and dirtcapacity using the tests described above with reference to Example 1.The results are as set out in Table 9 below.

                  TABLE 9                                                         ______________________________________                                               Pressure Drop at                                                       Filter 50 lpm clean water                                                                          Dirt Capacity                                                                             Absolute                                     No.    flow rate (mbar)                                                                            (g)         Rating                                       ______________________________________                                        16     255             15.5      less than 2 mm                               17     180           22          2.8 mm                                       ______________________________________                                    

It will be seen that both Filter No.16 and Filter No.17 have reducedpressure drop when compared with the "non-pleated" medium of Table 2.They also have high dirt capacity. In addition, they have the ability ofpositive zeta potential filters of removing from fluids by electrostaticattraction negatively charged particles that are much smaller than thepore size of the medium. Since most particles, including bacteria andviruses, are negatively charged, this is a useful ability.

Of course, the downstream layer could have negative zeta potential. Thiscan be useful in the removal of positively charged particles such asasbestos spores, aluminium hydroxide and cationic photoelectrolites.

As is known, polypropylene, when forming the filter medium may beprovided with positive zeta potential.

In Filter No.11 of Example 3, the central sleeve 22 may be formed of arectangular sheet of glass fibre membrane wrapped into a roll withopposite edges overlapping at least once. The downstream layer 23 may beformed by a roll formed from a sheet of nylon or polyester non-wovenfibres. This is shown in FIG. 12. The assembly is then pleated.

As seen in FIG. 13 the filter medium 24 of Example 4 may be combinedwith a downstream continuous or side sealed cylinder 25 of a thinmembrane such as the microporous nylon membranes sold by PallCorporation under the trade mark ULTIPOR. The combination is thenpleated.

By these measures composite pleated depth filtration media are providedwhich combine the lower pressure drops and higher dirt capacity ofpleated depth filter medium and also include the beneficialcharacteristics of the specific materials described.

In all these cases, upstream and downstream drainage nets may also beprovided, as shown in FIG. 11.

It has been found that the pleated depth filter media described abovewith reference to the drawings can have advantageous performance whenused in many applications. They are particularly advantageous when usedin the filtration of high viscosity fluids. In particular benefits havebeen found in the filtration of the following fluids:

1. Metal/metal oxide dispersions in organic solvents with viscosifyingadditives, as used in the manufacture of magnetic tapes

2. Solutions based on, for example, gelatin as used in photo emulsions

3. Fermenter broths

4. Ink solutions for ink jet printing machinery

5. Blood plasma

6. Monomers

7. Dialysis gels

8. Syrups used in the food and beverage industries

9. Viscous fluids used in the pharmaceutical industry, such as in thepreparation of cough syrups

10. Other high viscosity fluids such as concentrated HCL.

What is claimed is:
 1. A filter comprising a generally cylindrical depthfilter medium formed by at least one continuous sleeve of filter mediumwithout any side seal, said filter medium being provided with pleatsextending along the length of the filter medium and parallel to oneanother around the filter medium, an inner support core within thepleated depth filter medium and contacting inward ends of the pleats andan outer support cage contacting outer ends of the pleats.
 2. A filterelement according to claim 1 wherein the filter medium comprises aninner filtration layer arranged between upstream and downstream drainagelayers, the downstream drainage layer being formed integrally with atleast a part of the inner filtration layer and having an absolute ratingthat is greater than the absolute rating of the inner filtration layer.3. A filter element according to claim 2 and formed as a single pleatedcylindrical sleeve, an outer portion of the sleeve forming an upstreamdrainage layer, a central portion of the sleeve forming an innerfiltration layer and an inner portion of the sleeve forming thedownstream drainage layer.
 4. A filter element according to claim 3wherein the filter medium is formed by a fibrous mass of non-wovensynthetic polymeric micro-fibres, said micro-fibres being substantiallyfree of fibre-to-fibre bonding and secured to each other by mechanicalentanglement or inter-twining, said fibrous mass having a substantiallyconstant voids volume, the diameter of the fibrous structure decreasingfrom an exterior surface of the sleeve to the centre of the sleeve andthen increasing from the centre of the sleeve to an interior surface ofthe sleeve, to form successively said upstream drainage layer, saidinner filtration layer and said downstream drainage layer.
 5. A filterelement according to claim 4 wherein the mass of micro-fibres has aconstant smaller diameter in the inner filtration layer and has aconstant larger diameter in the downstream drainage layer.
 6. A filterelement according to claim 4 wherein the mass of micro-fibres has adiameter that increases through the inner filtration layer and thedownstream drainage layer.
 7. A filter element according to claim 4wherein the mass of micro-fibres has a constant smaller diameter in theinner filtration layer and an increasing diameter in the downstreamdrainage layer.
 8. A filter element according to claim 4 wherein themass of micro-fibres has a constant smaller diameter in the innerfiltration layer and has a constant larger diameter in the upstreamdrainage layer.
 9. A filter element according to claim 4 wherein themass of micro-fibres has a diameter that increases through the innerfiltration layer and the upstream drainage layer.
 10. A filter elementaccording to claim 9 wherein the mass of micro-fibres has a constantsmaller diameter in the inner filtration layer and has a diameter thatincreases through the upstream drainage layer to the surface of theupstream drainage layer.
 11. A filter element according to claim 6wherein a pleated cylinder of a microporous nylon membrane is provideddownstream of the filter medium, an exterior surface of the membranecontacting an interior surface of the filter medium.
 12. A filterelement according to claim 2 and formed as two pleated cylindricalsleeves arranged one inside the other with an outer surface of onesleeve in contact with an inner surface of the other sleeve, the outersleeve forming the upstream drainage layer and a part of the innerfiltration layer and the inner sleeve forming the remainder of the innerfiltration layer and the downstream drainage layer, the outer sleevebeing formed from a first material and the inner sleeve being formedfrom a second material.
 13. A filter element according to claim 12wherein the inner sleeve forming said remainder of the inner filtrationlayer and said downstream drainage layer comprises a fibrous mass ofnon-woven synthetic polymeric micro-fibres, said micro-fibres beingsubstantially free of fibre-to-fibre bonding and secured to each otherby mechanical entanglement or inter-twining, said fibrous mass having asubstantially constant voids volume, the diameter of the fibrousstructure increasing in a downstream direction so that a downstreamportion of said fibrous mass forms said drainage layer.
 14. A filterelement according to claim 13 wherein the mass of micro-fibres has aconstant smaller diameter in the remainder of the inner filtration layerand has a constant larger diameter in the downstream drainage layer. 15.A filter element according to claim 13 wherein the mass of micro-fibreshas a diameter that increases through the remainder of the innerfiltration layer and the downstream drainage layer.
 16. A filter elementaccording to claim 13 wherein the mass of micro-fibres has a constantsmaller diameter in the remainder of the inner filtration layer and anincreasing diameter in the downstream drainage layer.
 17. A filterelement according to claim 12 wherein the outer sleeve is formed by afibrous mass of non-woven synthetic polymeric micro-fibres, saidmicro-fibres being substantially free of fibre-to-fibre bonding andsecured to each other by mechanical entanglement or inter-twining, saidfibrous mass having a substantially constant voids volume, the diameterof the fibrous structure increasing in an upstream direction from saidpart of the inner filtration layer so that an upstream portion of saidfibrous mass forms said upstream drainage layer.
 18. A filter elementaccording to claim 17 wherein the mass of micro-fibres has a constantsmaller diameter in the part of the inner filtration layer and has aconstant larger diameter in the upstream drainage layer.
 19. A filterelement according to claim 17 wherein the mass of micro-fibres has adiameter that increases through the part of the inner filtration layerand the upstream drainage layer.
 20. A filter element according to claim17 wherein the mass of micro-fibres has a constant smaller diameter inthe part of the inner filtration layer and has a diameter that increasesthrough the upstream drainage layer to the surface of the upstreamdrainage layer.
 21. A filter element according to claim 12 wherein atleast one of said sleeves is formed from glass fibres bound together bya resin.
 22. A filter element according to claim 1 wherein an additionalpleated cylinder of a mass of nylon non-woven fibres is provideddownstream of said at least one sleeve, the additional cylinder havingan exterior surface in contact with an interior surface of said at leastone sleeve.
 23. A filter element according to claim 1 wherein ratio ofsurface area per unit length to depth of the filter medium is in therange from 20 to
 2000. 24. A filter element according to claim 23wherein the ratio of surface area per unit length to depth of the filtermedium is in the range from 40 to
 200. 25. A filter element according toclaim 1 wherein the filter element has an inner surface and an outersurface, said surfaces being covered by respective drainage nets.
 26. Afilter element comprising a generally cylindrical depth filter mediumincluding an inner filtration layer arranged between upstream anddownstream drainage layers, the downstream drainage layer being formedintegrally with at least a part of the inner filtration layer and havingan absolute rating that is greater than the absolute rating of the innerfiltration layer, the depth filter medium being provided with pleatsextending along the length of the filter medium and parallel to oneanother around the filter medium, an inner support core within thepleated depth filter medium and contacting inner pleats and an outersupport cage contacting outer pleats.
 27. A filter element according toclaim 26 in which the depth filter medium is formed by at least onecontinuous sleeve of filter medium without any side seal.
 28. A filterelement according to claim 26 and formed as a single pleated cylindricalsleeve, an outer portion of the sleeve forming an upstream drainagelayer, a central portion of the sleeve forming an inner filtration layerand an inner portion of the sleeve forming the downstream drainagelayer.
 29. A filter element according to claim 28 wherein the filtermedium is formed by a fibrous mass of non-woven synthetic polymericmicro-fibres, said micro-fibres being substantially free offibre-to-fibre bonding and secured to each other by mechanicalentanglement or inter-twining, said fibrous mass having a substantiallyconstant voids volume, the diameter of the fibrous structure decreasingfrom an exterior surface of the sleeve to the centre of the sleeve andthen increasing from the centre of the sleeve to an interior surface ofthe sleeve, to form successively said upstream drainage layer, saidinner filtration layer and said downstream drainage layer.
 30. A filterelement according to claim 29 wherein the mass of micro-fibres has aconstant smaller diameter in the inner filtration layer and has aconstant larger diameter in the downstream drainage layer.
 31. A filterelement according to claim 29 wherein the mass of micro-fibres has adiameter that increases through the inner filtration layer and thedownstream drainage layer.
 32. A filter element according to claim 29wherein the mass of micro-fibres has a constant smaller diameter in theinner filtration layer and an increasing diameter in the downstreamdrainage layer.
 33. A filter element according to claim 29 wherein themass of micro-fibres has a constant smaller diameter in the innerfiltration layer and has a constant larger diameter in the upstreamdrainage layer.
 34. A filter element according to claim 29 wherein themass of micro-fibres has a diameter that increases through the innerfiltration layer and the upstream drainage layer.
 35. A filter elementaccording to claim 34 wherein the mass of micro-fibres has a constantsmaller diameter in the inner filtration layer and has a diameter thatincreases through the upstream drainage layer to the surface of theupstream drainage layer.
 36. A filter element according to claim 29wherein a pleated cylinder of a microporous nylon membrane is provideddownstream of the filter medium, an exterior surface of the membranecontacting an interior surface of the filter medium.
 37. A filterelement according to claim 26 and formed as two pleated cylindricalsleeves arranged one inside the other with an outer surface of onesleeve in contact with an inner surface of the other sleeve, the outersleeve forming the upstream drainage layer and a part of the innerfiltration layer and the inner sleeve forming the remainder cf the innerfiltration layer and the downstream drainage layer, the outer sleevebeing formed from a first material and the inner sleeve being formedfrom a second material.
 38. A filter element according to claim 37wherein the inner sleeve forming said remainder cf the inner filtrationlayer and said downstream drainage layer comprises a fibrous mass ofnon-woven synthetic polymeric micro-fibres, said micro-fibres beingsubstantially free of fibre-to-fibre bonding and secured to each otherby mechanical entanglement or inter-twining, said fibrous mass having asubstantially constant voids volume, the diameter of the fibrousstructure increasing in a downstream direction so that a downstreamportion of said fibrous mass forms said drainage layer.
 39. A filterelement according to claim 38 wherein the mass of micro-fibres has aconstant smaller diameter in the remainder of the inner filtration layerand has a constant larger diameter in the downstream drainage layer. 40.A filter element according to claim 38 wherein the mass of micro-fibreshas a diameter that increases through the remainder of the innerfiltration layer and the downstream drainage layer.
 41. A filter elementaccording to claim 38 wherein the mass of micro-fibres has a constantsmaller diameter in the remainder of the inner filtration layer and anincreasing diameter in the downstream drainage layer.
 42. A filterelement according to claim 38 wherein the outer sleeve is formed by afibrous mass of non-woven synthetic polymeric micro-fibres, saidmicro-fibres being substantially free of fibre-to-fibre bonding andsecured to each other by mechanical entanglement or inter-twining, saidfibrous mass having a substantially constant voids volume, the diameterof the fibrous structure increasing in an upstream direction from saidpart of the inner filtration layer so that an upstream portion of saidfibrous mass forms said upstream drainage layer.
 43. A filter elementaccording to claim 42 wherein the mass of micro fibres has a constantsmaller diameter in the part of the inner filtration layer and has aconstant larger diameter in the upstream drainage layer.
 44. A filterelement according to claim 42 wherein the mass of micro-fibres has adiameter that increases through the part of the inner filtration layerand the upstream drainage layer.
 45. A filter element according to claim42 wherein the mass of micro-fibres has a constant smaller diameter inthe part of the inner filtration layer and has a diameter that increasesthrough the upstream drainage layer to the surface of the upstreamdrainage layer.
 46. A filter element according to claim 37 wherein atleast one of said sleeves is formed from glass fibres bound together bya resin.
 47. A filter element according to claim 26 wherein ratio ofsurface area per unit length to depth of the filter medium is in therange from 20 to
 2000. 48. A filter element according to claim 47wherein the ratio of surface area per unit length to depth of the filtermedium is in the range from 40 to
 200. 49. A filter element according toclaim 26 wherein the filter element has an inner surface and an outersurface, said surfaces being covered by respective drainage nets.
 50. Amethod of manufacturing a filter element comprising forming at least onecontinuous sleeve of depth filter medium without any side seal, pleatingthe sleeve to form a plurality of pleats extending along the length ofthe filter medium and parallel to one another around the filter medium,arranging an inner support core within the pleated depth filter mediumcontacting inward ends of the pleats and arranging an outer support cagein contact with outer ends of the pleats.
 51. A method according toclaim 50 and comprising forming the medium with an inner filtrationlayer arranged between upstream and downstream drainage layers, thedownstream drainage layer being formed integrally with at least a partof the inner filtration layer and having an absolute rating that isgreater than the absolute rating of the inner filtration layer.
 52. Amethod according to claim 51 and comprising forming the cylindricaldepth filter medium from a single pleated cylindrical sleeve, an outerportion of the sleeve forming an upstream drainage layer, a centralportion of the sleeve forming an inner filtration layer and an innerportion of the sleeve forming the downstream drainage layer.
 53. Amethod according to claim 51 and comprising forming two cylindricalsleeves, arranging one sleeve inside the other sleeve with an outersurface of one sleeve in contact with an inner surface of the othersleeve, and then pleating the sleeves, the outer sleeve forming theupstream drainage layer and a part of the inner filtration layer and theinner sleeve forming the remainder of the inner filtration layer and thedownstream drainage layer.
 54. A method according to claim 53 andcomprising forming two sleeves from a fibrous mass of non-wovensynthetic polymeric micro-fibres, said micro-fibres being substantiallyfree of fibre-to-fibre bonding and secured to each other by mechanicalentanglement or inter-twining, said fibrous mass having substantiallyconstant voids volume, the diameter of the fibrous structure decreasingin a downstream direction, inverting one of said cylindrical sleeves sothat an interior surface thereof becomes an exterior surface and theninserting the inverted sleeve into the other sleeve prior to pleating.55. A method of manufacturing a filter element comprising forming agenerally cylindrical depth filter medium including an inner filtrationlayer, arranged between upstream and downstream drainage layers, formingthe downstream drainage layer integrally with at least a part of theinner filtration layer and with an absolute rating that is greater thanthe absolute rating of the filtration layer, pleating the depth filtermedium to provide pleats extending along the length of the filter mediumand parallel to one another around the depth filter medium and thenproviding an inner support core within the pleated depth filter mediumand contacting inner pleats and an outer support cage contacting outerpleats.
 56. A method according to claim 55 and comprising forming thecylindrical depth filter medium from a single pleated cylindricalsleeve, an outer portion of the sleeve forming an upstream drainagelayer, a central portion of the sleeve forming an inner filtration layerand an inner portion of the sleeve forming the downstream drainagelayer.
 57. A method according to claim 55 and comprising forming twocylindrical sleeves, arranging one sleeve inside the other sleeve withan outer surface of one sleeve in contact with an inner surface of theother sleeve, and then pleating the sleeves, the outer sleeve formingthe upstream drainage layer and a part of the inner filtration layer andthe inner sleeve forming the remainder of the inner filtration layer andthe downstream drainage layer.
 58. A method according to claim 57 andcomprising forming two sleeves from a fibrous mass of non-wovensynthetic polymeric micro-fibres, said micro-fibres being substantiallyfree of fibre-to-fibre bonding and secured to each other by mechanicalentanglement or inter-twining, said fibrous mass having substantiallyconstant voids volume, the diameter of the fibrous structure decreasingin a downstream direction, inverting one of said cylindrical sleeves sothat an interior surface thereof becomes an exterior surface and theninserting the inverted sleeve into the other sleeve prior to pleating.